Environment suboxic

Black shales that are formed in an anoxic environment such as the Black Sea have a Mo isotope composition nearly identical to ocean water (Barling et al. 2001 Arnold et al. 2004 Nagler et al. 2005). Organic carbon rich sediments formed in suboxic environments have variable Mo/ Mo ratios intermediate between those of ocean water and oxic sediments (Siebert et al. 2003). Thus Mo isotope values in ancient black shales can be used as a paleo-oceanographic proxy of the oxidation state of the ocean, as for example has been discussed by Arnold et al. (2004) for the Proterozoic. Figure 2.25 summarizes natural Mo isotope variations. 

Certain smaller suboxic environments within larger estuarine systems also show unbioturbated sections of sediments. Because of their generally rapid rate of accumulation, the cosmogenic nuclide Be (53 d half-life) may be used in addition to °Pb for sediment accumulation rate assessment for the more recently deposited sediments. Similarly the pattern of bomb-produced Cs in sediments can serve as a chronometer. In addition, radiocarbon ages commonly can be obtained from calcareous fractions in the accumulating sediment. All these approaches were used at the FOAM site in Long Island Sound by Krishnaswami et al. (1984). 

Source Proposed by Berner (1981b) with minor modifications. The suboxic environment is described by Anderson et al. (1994). See also Kent et al. (1994),... 

Based on the pE value, redox environments are classified as follows (a) pE > 7 indicates an oxic environment, (b) at pE values between 2 and 7, the environment is considered suboxic, and (c) pE < 2 indicates an environment considered anoxic. The occurrence of redox reactions in the subsurface environment is limited by the decomposition and reduction of water ... 

Murray JW, Codispoti LA, Friederich GE (1995) Oxidation-reduction environments the suboxic zone in the Black Sea. In Huang CP, O Melia CR, Morgan JJ (eds) Aquatic chemistry interfacial and interspecies processes. Adv Chem Ser. ACS, Washington,... 

A more serious consideration for shelf and coastal dynamics would be the creation of significant dams along the mainstem of the Amazon River. Such structures certainly would reduce the delivery of sediment to the continental shelf. Benthic processes such as fluid mud formation and suboxic diagenetic reaction balances could be altered by these activities. Downdrift coastal mud banks and coastal mangrove environments also could be affected. 

Vogel et al., 1987). It seems likely to us that there is a biological transformation of these compounds by marine bacteria, particular as marine bacteria can transform CHaBr (see Section 6.03.3.3.2). Not only are these compounds likely to be removed from oceanic and coastal waters under anoxic and suboxic conditions, but given that compounds such as CH4 and N2O are thought to be produced in suboxic micro-environments within the water column (see Section 6.03.3.2.9), it seems reasonable to assume that the same sites might be areas of chlorofluorocarbon removal. 

Aside from anoxic or suboxic basins, the other marine environment suitable for radioactive geochronometry is salt-marsh deposits. As sea level has risen over the past 100 years, salt marshes have kept up by vertical growth of a vegetated framework that supports sediment accumulation. In addition, since high salt marshes are inundated by seawater only —5% of the year, the surface becomes an accumulator of atmospherically derived species including °Pb. The radioactive decay of °Pb can then be used to determine the age of levels in the salt marsh and thereby the accumulation rate of the salt marsh and its components. Since the salt-marsh vertical growth depends on the rise in sea level, the °Pb chronometer becomes a proxy for the rate of rise... 

Sobolev D. and Roden E. E. (2001) Suboxic deposition of ferric iron by bacteria in oppposing gradients of Fe(II) and oxygen at circumneutral pH. Appl. Environ. Microbiol. 67, 1328-1334. 

Haury, V., Jann, S., Kofod, M., Scholz, C., and Isenbeck-Schroter, M., 2000, Redox-induced species distribution of arsenic in a suboxic groundwater environment - column experiments in Proceedings of the International Conference on Groundwater Research, Copenhagen, Denmark, p. 197-198. 

Suboxic metabolism, which includes nitrate, manganese and iron reduction, does not generally seem to form a discrete layer due to the relatively low levels of nitrate available under natural conditions and the insoluble nature of manganese oxyhydroxides and iron oxides. These reactions appear to be restricted, where they are apparent at all, to micro-environments, but since, of the reduction reactions in Table 6.1, they are the only ones to generate alkaline conditions, they may be important for the precipitation and preservation of carbonate minerals. 

As in the oxic zone, the 6 C values of suboxic carbonates in continental environments are controlled by the 6 C of atmospheric carbon and by the oxidation of terrestrial organic matter in the soil profile, whereas the 6 0 values are mainly controlled by latitude and climatic conditions. 

Siderite precipitates from reducing, non-sulphidic pore waters that evolve in the suboxic and microbial methanogenesis zones of all depMDsitional environments. These geochemical conditions occur in organic-rich sediments containing appreciable amounts of reactive iron minerals and in which the pore waters are S04 -poor meteoric or brackish (Postma, 1982). 

Within the ODZ, nitrous oxide (N20), another major intermediate of denitrification (and a byproduct of nitrification), shows a trend of variability quite different from that of N02 (Fig. 6.15), but similar to that observed in the ODZs of the Pacific Ocean (Codispoti Christensen, 1985). That is, N20 concentration generally increases non-linearly with the depletion in 02 until the environment turns reducing thereafter, concomitant with the accumulation of secondary N02, a rapid fall in N20 concentration takes place. Accordingly, the SNM is characterized by a minimum in N20 concentration (<10nM), whereas the oxic-suboxic interfaces are characterized by peak N20 levels exceeding 50nM (Law Owens, 1990 Naqvi Noronha, 1991). Attempts have been made to evaluate the relative importance of nitrification, denitrification and coupling between the two processes as pathways for N20 production by... 

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